The week is the one unit with nothing behind it
A day is the Earth turning once. A MONTH is roughly the Moon going round us. A year is us going round the Sun. A WEEK is seven days because somebody said so — nothing in the sky takes seven days to do anything.
It seems to have come from Babylon, where seven objects were visible moving against the stars, and it spread from there across most of the world. It has also been attacked twice. Revolutionary France imposed a ten-day week in 1793, which gave workers one rest day in ten and was abandoned inside a decade. The Soviet Union tried five-day and then six-day weeks in the 1930s to keep factories running continuously, gave up in 1940, and went back to seven.
September really does mean seven
SEPTEMBER, OCTOBER, NOVEMBER and DECEMBER contain the Latin for seven, eight, nine and ten, and sit in the ninth, tenth, eleventh and twelfth places. The numbering is not broken so much as out of date.
The old Roman year began in MARCH, ran for ten months and then stopped, leaving a nameless stretch of winter that nobody bothered counting because there was no farming to organise. JANUARY and FEBRUARY were eventually added to fill the gap, everything shifted by two, and the four numbered months were never renamed. Latin lessons have been explaining this ever since.
Wednesday is a god in disguise
The Romans named the days after the sun, the moon and five planets. Germanic speakers took the system and swapped in their own gods wherever one looked comparable. English and French have disagreed about the middle of the week ever since, while agreeing about its shape.
TUESDAY went to Tiw, a war god, where French kept Mars and got mardi. WEDNESDAY belongs to Woden — spelled with letters nobody pronounces any more — against Mercury and mercredi. THURSDAY is Thor's, matched against Jupiter in jeudi, and FRIDAY is Frigg's, against Venus in vendredi. Only SATURDAY held on to its Roman owner, Saturn, while SUNDAY and MONDAY stayed with the sun and moon in both languages.
February is short because it was last
The month that gets cut is the one that used to sit at the end of the year, and a shortened FEBRUARY is what is left of the winter gap. Romans also disliked even numbers, so the calendar was arranged into months of 29 and 31 days with one unlucky leftover, and February drew it.
The popular story that Augustus stole a day from it so that AUGUST would not be shorter than JULY is a much later invention, repeated because it is a good story. February's problem is simply that it has always been where the spare days were kept, which is also why the extra leap day gets stuffed in there every four years.
The spelling traps
All seven days are in the grid, MONDAY through SUNDAY, along with the months from JANUARY and FEBRUARY, through MARCH, APRIL, JUNE, JULY and AUGUST, to SEPTEMBER, OCTOBER, NOVEMBER and DECEMBER, plus WEEK and MONTH.
Two of them cause most of the trouble. WEDNESDAY hides a whole syllable that stopped being said centuries ago, and FEBRUARY has an r most people skip. Both are far easier to spell after you have hunted the letters out one at a time, which is a reasonable argument for searching for the hard words rather than the easy ones.